1. Technical Field
The invention relates to an RF plasma etch reactor, and more particularly to such a reactor employing an internal inductive coil antenna and electrically conductive chamber walls.
2. Background Art
A typical inductively coupled plasma etch reactor of the type currently available is depicted in FIG. 1. This reactor has a vacuum chamber 10 surrounded by an inductive coil 12. A workpiece 14, usually a semiconductor wafer, is supported inside the chamber 10 on a pedestal 16. An inductive coil antenna 12 is wound around the outside of the chamber 10 and connected to a radio frequency (RF) power generator 18 through an impedance matching network 20 to provide RF power into the chamber. In addition, a bias RF power generator 22 and associated impedance matching circuit 24 is connected to the pedestal 16 and used to impose a bias on the workpiece 14. The chamber walls 30 are composed of an electrically insulating material, typically quartz or ceramic, so as to minimize attenuation of the RF power coupled into the chamber 10. Underlying the insulative chamber walls 30 and surrounding the pedestal 16 is a portion 34 of the chamber constructed of a conductive material. This conductive portion 34 is electrically grounded and serves as the ground for the RF power supplied to the pedestal 16. There are also cooling channels 32 formed within the conductive portion 34. Coolant fluid is pumped through the channels 32 to transfer heat away from the interior of the chamber 10 so that the chamber temperature can be maintained at a particular level desired for the etch process being performed. The exterior of the chamber walls 30 are also cooled for the same reason. However, as insulative materials such as quartz and ceramic cannot be easily formed with internal cooling channels, the exterior surface of the walls 30 are cooled, typically by forced air convection methods. Etchant gas is introduced into the chamber 10 through gas injection ports 26. A vacuum pump 28 evacuates the chamber 10 to a desired chamber pressure.
In operation, an etchant gas is introduced into the interior of the chamber 10 and RF power inductively coupled from the coil 12 generates a plasma within the chamber. The plasma produces etchant species (e.g. ions and radicals) from the etchant gas which are used to etch the workpiece 14. A key component of anisotropic etching processes is the bombardment of the workpiece 14 with ions produced in the plasma. The energy and directionality exhibited by the ions and their density within the plasma are important factors which, to a large part, determine the quality of the resulting etched workpiece 14. These factors substantially determine etch uniformity, etch rate, photoresist selectivity, the straightness of the etch profile, and the smoothness of etch feature sidewalls. For example, a high plasma ion energy at the surface of the workpiece 14 is desirable so as to prevent isotropic etching and to maximize the etching rate. However, ion energy which is too high will produce poor etching results, such as high photoresist loss, and can cause damage to the devices being formed on the workpiece 14. Therefore, the plasma ion energy is ideally kept relatively near but below a threshold at which the etch quality begins to deteriorate significantly and/or where device damage becomes unacceptable. Similarly, a high plasma ion density is desirable so as to maintain a high etch rate. Essentially, the more ions there are, regardless of their energy, the faster the workpiece 14 is etched.
In the inductively coupled reactor of FIG. 1, the plasma ion density is substantially controlled by the amount of RF power coupled into the chamber via the coil 12. For the most part, the more power coupled, the higher the plasma ion density. Thus, in most cases, the plasma ion density can be held to a desired level by selecting the appropriate amount of RF power to be supplied by the RF power generator 18 to the coil 12. The RF power coupled into the chamber by the coil 12, however, does not significantly affect the plasma ion energy at the surface of the workpiece 14. Control of the ion energy at the surface of the workpiece is conventionally accomplished by capacitively coupling RF power into the chamber via the to the pedestal 16 using the bias RF power generator 22. Ideally, the bias power supplied to the pedestal 16 will not significantly affect the ion density produced in the chamber 10, thereby decoupling the control of ion density and ion energy.
The plasma ion energy controlled by the bias RF power applied to the pedestal 16 is, however, affected by the ratio of the surface area of the pedestal to the surface area of the grounded portion 34 of the chamber. The pedestal 16 acts as the cathode and the grounded portion 34 serves as the anode to form a capacitively coupled circuit. Since the majority of the interior surface of the chamber 10 is formed by the insulative chamber walls 30 to maximize the inductive coupling of power into the chamber from the coil 12, the surface area associated with the grounded portion 34 is necessarily limited, and typically not too much larger than that of the pedestal 16. An ion energy control problem results because the surface areas of the grounded portion 34 and the pedestal 16 are too close in size in a conventional inductively coupled etch reactor. When the surface area of the pedestal 16 is less than that of the grounded portion 34, the average voltage (often referred to as the DC bias voltage) at the surface of the workpiece 14 is negative. This average negative voltage is employed to draw the positively charged ions from the plasma to the workpiece 14. However, if the surface area of the pedestal 16 is only slightly smaller than the surface area of the grounded portion (as is typically the case in a conventional inductively coupled plasma etch reactor), the average negative voltage at the surface of the workpiece 14 is relatively small. This small bias voltage results in a weak attracting force and so a relatively low average ion energy. A higher negative bias voltage value than can typically be obtained using a conventional inductively coupled plasma etch reactor is necessary to optimize the plasma ion energy so as to ensure the maximum etch rate and no significant damage to the devices being formed on the workpiece 14. Ideally, the surface area of the grounded portion 34 would be sufficiently large in comparison with that of the pedestal 16 so as to produce the maximum possible negative average voltage at the surface of the workpiece 14, i.e. one half the peak to peak voltage.
The previously-described inductively coupled etch reactor has in the past been used to etch aluminum from the surface of the workpiece 14. This etching process produced byproducts comprising mostly aluminum chlorides (AlCl.sub.x) and fragments of photoresist which tend to deposit on the walls of the reactor chamber 10. The byproducts of an aluminum etch have no significant effect on the plasma characteristics (e.g. plasma ion density and energy) because they are almost totally non-conductive. However, it is also desirable to etch other metals from the surface of a workpiece 14, such as copper (Cu), platinum (Pt), tantalum (Ta), rhodium (Rh), and titanium (Ti), among others. Etching these metals presents a problem when using the conventional etch reactor of FIG. 1 because the etching by-products of these metals tend to be conductive. Thus, a conductive coating forms on the chamber walls. This conductive coating has the effect of attenuating the RF power coupled into the chamber by the coil 12. The coil 12 produces a magnetic field which results in power being coupled into the chamber. When the interior surface of the chamber under the coil 12 is coated with a conductive material, eddy currents are produced in this material, thereby attenuating the magnetic field to some extent and reducing the amount of power coupled into the interior of the chamber 10. As the conductive coating builds in thickness over successive etch processes, the attenuation progressively increases and the power coupling into the plasma progressively decreases. It has been found that a 10 to 20 percent decrease in power coupled into the plasma occurs after processing 100 workpieces. In addition, the conductive coating can electrically couple to the grounded anode portion 34 of the chamber, thereby effectively increasing the anode area. This increase in anode area in turn tends to increase the previously mentioned negative DC bias voltage. The change in the bias voltage due to the altered effective anode area results in an unexpected increase in the capacitive coupling of RF power from the pedestal.
The progressive reduction of inductively coupled RF power and increase in capacitively coupled RF power have detrimental effects on the etching process. For example, the plasma ion density is lowered due to the decrease in inductively coupled RF power and the plasma ion energy is increased due to the increase in capacitively coupled power. As the RF power levels are typically set prior to the etching process to optimize plasma ion density and energy, any change could have an undesirable impact on etch quality. The changes in power coupling caused by conductive etch by-products coating the chamber 10 also affect other etch process parameters and plasma characteristics, as well. For instance, the photoresist selectivity is lowered, etch stop depths are reduced, and ion current/energy distribution and the etch rate are adversely affected. These changed parameters and characteristics result in different, and often unacceptable workpiece etch characteristics (such as poor photoresist selectivity, poor etch rate uniformity or etch rate shift, and device damage). It has been found that even after only two or three workpieces 14 have been etched, unwanted changes in the etch profile can be observed. In addition to the detrimental effects on the etch process parameters and plasma characteristics, it has also been found that the reduced inductive coupling of RF power into the chamber 10 causes problems with igniting and maintaining a plasma.
Of course, the decrease in inductively coupled power could be compensated for by increasing the RF power supplied to the coil 12. Similarly, the increase in capacitively coupled power can be compensated for by decreasing the RF power supplied to the pedestal 16. In addition, the chamber walls can be cleaned more often than would typically be necessary when etching materials producing non-conductive by-products such as aluminum. However, these types of work-arounds are generally impractical. A user of an etch reactor typically prefers to set the respective RF power levels in accordance with a so-called "recipe" supplied by the reactor's manufacturer. Having to deviate from the recipe to compensate for the conductive deposits would be unacceptable to most users. Further, it is believed that the aforementioned detrimental effects will be unpredictable, and therefore, the required changes in the RF power settings needed to compensate could not be predetermined. Thus, unless the user employs some form of monitoring scheme, the required compensating changes in RF power input would be all but impossible for a user to implement. Realistically, the only viable solution would be to clean the chamber frequently, perhaps as often as after the completion of each etch operation. However, this increase in the frequency of cleaning (for example, over that necessary when etching aluminum) would be unacceptable to most users as it would lower throughput rates and increase costs significantly.
Another drawback associated with a conventional inductively coupled etch reactor, such as the one depicted in FIG. 1, is that the structure places limitations on power deposition and etchant species diffusion within the chamber 10. Power deposition in an etch reactor's chamber 10 concerns the distribution of power within the chamber's interior. For example, the regions 11 designated by dashed lines in FIG. 1, exhibit a high level of power deposition owing to their proximity to the coil 12. Whereas, the power deposition away from these regions 11, such as near the workpiece 14, is much lower. However, in many applications, it is desirable that the region of the chamber immediately adjacent the exposed surface of the workpiece 14 exhibit a high power deposition. For example, a high power deposition near the exposed surface of the workpiece 14 may be advantageously used to create a high plasma ion density in that region. Granted, the shape of the chamber might be changed to move the coil 12, and so the region of high power deposition, nearer to the workpiece 14. A variety of chamber shapes are known. For example, dome-shaped chambers are sometimes employed wherein the coil wraps around the outside also forming a dome shape. However, there are limits to how the chamber can be shaped in an attempt to bring the regions of high power deposition to the most advantageous location in relation to the workpiece. These limits derive from the fact that the shape of the chamber also has a significant impact on the characteristics of the plasma and the etch processing parameters associated therewith. Thus, a compromise must be struck between the shape of the chamber and the desired power deposition pattern therein. Typically, this precludes optimizing the power deposition within the chamber.
The other factor mentioned above is etchant species diffusion. This term refers to the tendency for etchant species to migrate from areas of high concentrations, such as a region having a high power deposition where they tend to be formed in great quantities, to areas of lower concentrations. The diffusion patterns are dependent upon the particular type of etchant species involved, and can vary significantly from one to another. Thus, it is possible to influence the make-up of the plasma adjacent the exposed surface of the workpiece 14 by tailoring the power deposition profile in the chamber to take advantage of the diffusion characteristics of the etchant species formed in the plasma. Consequently, it is still feasible to have regions 11 of high power deposition remote from the exposed surface of the workpiece 14, while creating the desired plasma characteristics in the region adjacent this surface. However, a problem arises when the particular species that is desired to be diffused to the region adjacent the workpiece 14 is of a type having a relatively short life span, so short that it no longer exists by the time mere diffusion processes would have brought it into the region adjacent the workpiece. Again, employing a differently shaped chamber could assist in bringing the high power deposition regions 11 closer to the workpiece 14, and thereby making it more likely the desired short-lived etchant species reach the workpiece while still viable. However, this reshaping must be balanced against the effect the chamber shape has on the plasma characteristics associated therewith. It has been found that the chamber cannot be reshaped to the extent necessary to ensure many known short-lived etchant species are present at the surface of the workpiece 14. For example, employing the conventional reactor configuration shown in FIG. 1, and a typical etchant gas such as chlorine, short-lived species such as Cl.sup.+ and Cl.sub.2.sup.+ ions in excited states which are formed in the regions 11 of high power deposition will not diffuse into the region adjacent the workpiece 14 prior to becoming extinct.
Yet another drawback associated with a conventional inductively coupled etch reactor, such as the one shown in FIG. 1, involves the cooling of the walls of the chamber 10. Etching processes are typically only stable and efficient if the chamber temperature is maintained within a narrow range. However, formation of the plasma generates heat which can raise the chamber temperature above the required narrow range. Consequently, it is desirable to remove heat from the chamber 10 in order to maintain the optimum temperature range associated with the etch process being performed. As mentioned previously, this is typically done by flowing coolant fluid through the cooling channels 32 formed within the conductive portion 34 of the chamber 10 and flowing air over the exterior of the insulative chamber walls 30. A problem arises in that the electrically insulative materials, such as quartz or ceramic, typically used to form the chamber walls also exhibit a low thermal conductivity. Thus, the chamber walls are thermally insulative and do not make an ideal heat transfer medium for picking up heat from the interior of the chamber 10 and dumping it into the air flowing over the exterior of the walls. As a result, the chamber temperature tends to fluctuate more than is desired in the region adjacent the insulative chamber walls because the heat transfer from the chamber 10 is sluggish. Often the temperature fluctuations exceed the aforementioned narrow range required for efficient etch processing. In addition, these excessive fluctuations can cause another problem. As discussed previously, etch by-products will tend to deposit on the chamber walls during the etch process. In attempting to control the chamber temperature by air cooling the insulative chamber walls 30, the chamber wall temperature and the layer of etch by-product formed on the interior surface thereof, tends to cycle. This cycling causes thermal stresses within the layer of etch by-product material which result in cracks and pieces of the material flaking off the wall and falling into the chamber. The loose deposit material can contaminate the workpiece, or it can settle at the bottom of the chamber thereby requiring frequent chamber cleaning.
It is often desirable to inject etch process gas directly into the regions having the highest power deposition. In the conventional etch reactor shown in FIG. 1, these regions 11 are immediately adjacent the coil 12. However, pathways to accommodate the gas injection ports 26 cannot be formed in the chamber walls adjacent these areas of high power deposition without physically interfering with the induction coil 12. Thus, the gas has to be injected either in a void at the top of the coil 12 or below the coil. Granted the flow of gas from these ports 26 can be directed toward the regions 11 of high power deposition, but it is has been found that this method is insufficient to ensure the optimum concentration of etchant gas in these regions.
A conventional inductively coupled RF plasma etch reactor also must be operated at relatively low pressures (e.g. below 100 mTorr) in comparison to a conventional capacitively couple etch reactor (which can operated up to 10 Torr). Often etch processes will work best if performed at the higher pressures beyond the range of a conventional inductively coupled plasma reactor. In addition, relatively high RF power levels must be supplied to the coil antenna in order to overcome the impedance created by the insulative chamber walls and still provide enough power to the chamber to ignite and sustain a plasma therein. Accordingly, a large-capacity RF power supply must be employed.
Accordingly, what is needed is an RF plasma etch reactor that is unaffected by conductive etch by-products which deposit on the interior of the chamber. In addition, it is desirable that such an etch reactor be capable of producing a self-bias voltage which will optimize the ion bombardment at the surface of the workpiece, as well as allowing the tailoring of the power deposition within the chamber without the restrictions imposed by the shape of the chamber walls. Further, the etch reactor would preferably have chamber walls which can be maintained within a narrow temperature range which optimizes etch processing and prevents the flaking of deposits. It is also desirable that the gas injection inlets be placeable anywhere on the chamber walls. And finally, the etch reactor would preferably be operable at pressures in excess of about 100 mTorr and using an RF power level less than that required to be supplied to the coil antenna of a conventional inductively coupled RF plasma etch reactor.